# Faster than C

Dale
Mentor
My point was that we can say it's inconsistent with the Standard Model, but saying it's impossible goes too far.
What is impossible is the combination of FTL, relativity, and causality.

Staff Emeritus
2019 Award
True. We could just suppose that the tachyon link would be perceived to fail if it wasn't well contained within the same reference frame.
Yes, but you have just created a preferred frame, and in doing so violated SR.

Yes, but you have just created a preferred frame, and in doing so violated SR.
OK. I was thinking of the two ends of the communication link as being essentially "at rest" with wrt each other. Are you saying that the idea of two observers being "at rest" wrt each other cannot apply across a space-like interval under SR? Tachyons are a consequence of SR.

DrGreg
Gold Member
OK. I was thinking of the two ends of the communication link as being essentially "at rest" with wrt each other. Are you saying that the idea of two observers being "at rest" wrt each other cannot apply across a space-like interval under SR? Tachyons are a consequence of SR.
Suppose we postulate a tachyonic comms system as consisting of a transmitter & receiver at rest relative to each other with a fixed transmission speed that's faster than light within the rest frame. We need two such systems A and B, moving relative to each other.
1. transmitter A sends tachyonic message to receiver A
2. receiver A relays the message to transmitter B by radio (at the speed of light) which is close by at the time
3. transmitter B relays the message tachyonically to receiver B
4. receiver B relays the message to transmitter A by radio (at the speed of light) which is close by at the time

If the distances and velocities are chosen appropriately, transmitter A can receive the message before transmitter A sent it.

(Credit to Fredrik who gave this same argument not long ago, but in a brief search I couldn't find the message where he gave it.)

What is impossible is the combination of FTL, relativity, and causality.
I think this is the main point. They are an unholy trinity.

I recently read a book called "The God Effect" that discussed instantaneous communication via entanglement as a possibility- but it the conclusion was rather pessimistic. You would definitely find the novel interesting.